Natural gas produced from natural gas wells contains sulfur compounds such as hydrogen sulfide, other sulfides, and thiophene. Crude oil from oil wells also contains sulfur compounds. Hydrocracking of crude oil produces hydrogen sulfide along with light gases. Since sulfur compounds turn into anticatalysts during reaction processes and into sulfur oxides (SOx) when burned, sulfur components are usually separated and recovered and not discharged directly. Because natural gas wells with low sulfur compound contents have been exploited actively first, natural gas currently produced from the remaining natural gas wells has sulfur compound concentrations as high as about 40% by weight.
Hence, gas plants and oil refinery plants are equipped with sulfur recovery units (hereinafter “SRUs”) that absorb and separate hydrogen sulfide from natural gas with high hydrogen sulfur contents or from light gas and recover elemental sulfur. Recent SRUs have grown in size to accommodate the increase in sulfur compound concentration.
An SRU includes a reaction furnace configured to carry out a high-temperature Claus reaction. According to a high-temperature Claus reaction, hydrogen sulfide is reacted with oxygen in air at high temperatures to obtain elemental sulfur (S2) and water (H2O) from hydrogen sulfide (H2S). The high-temperature Claus reaction is carried out at a temperature over 850° C.
The reaction furnace also serves as a waste heat boiler (WHB) in order to effectively use reaction heat generated by the Claus reaction. In a WHB, the reaction gas is subjected to primary cooling and heat is recovered as high-pressure steam. Since the reaction gas contains remaining sulfur compounds such as hydrogen sulfide and sulfur oxides, an SRU is further equipped with a reactor configured to heat the reaction gas, which has been subjected to primary cooling, with low-pressure steam to induce a catalytic Claus reaction and convert unreacted hydrogen sulfide into elemental sulfur. Sulfur in the gas that has reacted in the reaction furnace or reactor is cooled to about 140° C. and is recovered as liquid sulfur. An SRU process is, for example, disclosed in Patent literature No. 1.
Since the reaction gas has a high temperature as discussed above, the pipes and devices constituting the SRU expand due to heat as the temperature rises. However, since the displacement thereof is restrained, thermal stresses are generated. Accordingly, measures against thermal expansion have been taken in the pipes and devices constituting the SRU, such as increasing the strength of surrounding portions of nozzles of the devices so that the surrounding portions have a sufficient strength against thermal stresses generated in the nozzle section or installing pipe structures that absorb thermal expansion or contraction so as to decrease thermal stresses generated in the pipe and the nozzle section. Such pipe structures are installed in the middle of pipes and bent into a ring shape so as to absorb the expansion or contraction generated by the temperature difference and are thus called “expansion loops”.
FIG. 1 is a diagram illustrating an example of a typical SRU. The SRU in FIG. 1 includes a reaction furnace 1010 for mixing and incinerating hydrogen sulfide (H2S) and air to carry out a high-temperature Claus reaction, a condenser 1020 that cools the reaction gas, and a pipe 1030 that connects the reaction furnace 1010 and the condenser 1020. The pipe 1030 includes an expansion loop 1040. The reaction furnace 1010, the condenser 1020, and the pipe 1030 thermally expand when heated to high temperatures and thermal stresses are generated as a result. Deformation caused by the thermal expansion is absorbed by elastic deformation of the expansion loop 1040 of the pipe 1030 between the reaction furnace 1010 and the condenser 1020. This decreases the thermal stresses applied to the nozzles of devices. In order to avoid excessive deformation due to thermal stresses, the nozzles of the reaction furnace 1010 and the condenser 1020 are designed to have a thickness and strength sufficient to withstand the thermal stresses. In order to ensure there be sufficient elastic deformation in a direction of the thermal expansion or thermal contraction, the expansion loop is provided in a direction orthogonal to the direction of thermal expansion or thermal contraction. In FIG. 1, only one expansion loop is provided. However, two or more expansion loops are desirably provided when the displacement caused by thermal expansion or thermal contraction of the pipe is expected to be large.